Wilmington
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilmington clear sky chart from 2005-11-06 to 2025-05-26.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	113989
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 44%	115256
Transparency	Day	23% 42% 7% 14% 11% 2	117580
	Night	24% 40% 7% 15% 11% 3	119827
Seeing	Day	17% 10% 26% 26% 15% 6%	15667
	Night	18% 8% 30% 32% 12% 1	25085

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 5% 2 5% 4 5% 4 4 5% 6% 49%	3597
	11th to 20th	Day	14% 8% 3 7% 5% 5% 4 3 4 5% 43%	3753
	21st to end	Day	18% 7% 2 5% 4 4 4 3 4 5% 44%	4466
February	1st to 10th	Day	15% 8% 2 6% 4 5% 5% 4 6% 7% 39%	1800
	11th to 20th	Day	17% 7% 2 6% 4 3 5% 4 5% 6% 40%	1893
	21st to end	Day	17% 10% 3 6% 3 4 3 2 5% 6% 39%	1650
March	1st to 10th	Day	20% 4 2 4 3 3 4 3 4 7% 46%	4540
	11th to 20th	Day	13% 7% 2 5% 4 4 4 3 5% 7% 47%	4980
	21st to end	Day	18% 6% 2 6% 3 4 4 3 5% 6% 43%	5758
April	1st to 10th	Day	16% 6% 2 6% 5% 4 4 4 5% 7% 41%	2400
	11th to 20th	Day	14% 7% 1 6% 4 5% 4 3 6% 7% 43%	2374
	21st to end	Day	16% 5% 2 6% 4 4 4 4 6% 8% 41%	2400
May	1st to 10th	Day	29% 8% 3 5% 3 4 4 3 5% 7% 29%	4150
	11th to 20th	Day	24% 8% 3 7% 5% 5% 4 4 5% 7% 28%	4538
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 9% 45%	4907
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 8% 9% 39%	2660
	21st to end	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 7% 5% 8% 11% 39%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 11% 39%	4440
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 8% 9% 32%	4665
August	1st to 10th	Day	17% 6% 3 6% 5% 4 6% 4 7% 8% 33%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 7% 10% 41%	3741
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 47%	4180
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 6% 44%	2280
	11th to 20th	Day	12% 5% 1 5% 3 4 4 3 5% 8% 50%	2262
	21st to end	Day	19% 8% 3 7% 4 4 4 3 6% 5% 38%	2071
October	1st to 10th	Day	16% 6% 2 5% 5% 4 4 3 5% 6% 44%	1900
	11th to 20th	Day	16% 4 2 4 4 3 4 3 5% 7% 49%	1900
	21st to end	Day	17% 6% 2 5% 4 4 3 3 4 5% 47%	2048
November	1st to 10th	Day	12% 6% 2 5% 3 2 3 3 4 6% 53%	1761
	11th to 20th	Day	16% 5% 1 5% 3 3 4 3 6% 7% 48%	1800
	21st to end	Day	15% 6% 2 5% 4 3 4 3 5% 6% 48%	1665
December	1st to 10th	Day	13% 7% 2 5% 4 4 4 3 5% 6% 46%	2600
	11th to 20th	Day	20% 9% 2 4 3 3 3 3 4 6% 43%	2986
	21st to end	Day	16% 6% 2 6% 4 4 4 5% 6% 8% 41%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 49%	6349
	11th to 20th	Night	15% 7% 3 5% 3 3 3 2 4 5% 51%	6115
	21st to end	Night	18% 7% 2 6% 4 4 3 3 5% 5% 44%	6490
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	20% 6% 3 5% 4 4 3 3 5% 6% 42%	2392
	21st to end	Night	17% 7% 2 5% 4 4 3 4 6% 6% 41%	1980
March	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 4 6% 46%	5283
	11th to 20th	Night	11% 8% 2 5% 4 4 3 3 4 5% 52%	4979
	21st to end	Night	15% 8% 2 6% 4 3 4 3 5% 5% 44%	5174
April	1st to 10th	Night	17% 7% 3 6% 4 4 4 3 5% 7% 41%	2000
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 7% 43%	1975
	21st to end	Night	18% 5% 2 5% 4 4 4 3 5% 7% 44%	2000
May	1st to 10th	Night	23% 9% 3 8% 5% 4 4 4 5% 8% 30%	3040
	11th to 20th	Night	24% 9% 3 7% 5% 4 4 4 5% 7% 29%	2575
	21st to end	Night	14% 6% 2 5% 4 3 4 4 6% 9% 43%	2784
June	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 6% 8% 44%	1520
	11th to 20th	Night	10% 6% 3 7% 6% 5% 4 4 6% 10% 39%	1520
	21st to end	Night	10% 5% 2 5% 5% 4 4 5% 7% 11% 42%	1520
July	1st to 10th	Night	7% 6% 2 8% 5% 6% 6% 4 7% 10% 38%	2520
	11th to 20th	Night	11% 5% 2 6% 6% 5% 5% 5% 8% 11% 36%	2520
	21st to end	Night	14% 8% 3 7% 5% 5% 4 5% 7% 8% 36%	2885
August	1st to 10th	Night	20% 5% 2 8% 4 4 5% 4 5% 9% 34%	2825
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 3 6% 7% 43%	3054
	21st to end	Night	11% 5% 1 5% 4 4 5% 3 5% 7% 50%	3476
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 4 7% 46%	1900
	11th to 20th	Night	13% 6% 2 4 3 2 3 2 5% 7% 53%	1917
	21st to end	Night	20% 7% 2 6% 3 4 4 3 5% 8% 38%	2104
October	1st to 10th	Night	20% 6% 3 6% 4 3 4 3 4 5% 43%	2280
	11th to 20th	Night	16% 4 2 5% 3 3 3 2 5% 6% 51%	2280
	21st to end	Night	16% 5% 1 6% 2 3 3 3 4 6% 50%	2548
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 5% 53%	2537
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 4 5% 48%	2600
	21st to end	Night	13% 5% 2 4 4 4 5% 4 6% 7% 46%	2730
December	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 6% 48%	4580
	11th to 20th	Night	23% 6% 2 5% 3 3 3 3 5% 7% 41%	5208
	21st to end	Night	18% 7% 2 5% 4 4 4 4 6% 6% 40%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 27% 7% 19% 20% 3	3598
	11th to 20th	Day	33% 27% 4 18% 13% 6%	3784
	21st to end	Day	31% 28% 11% 17% 11% 2	4466
February	1st to 10th	Day	18% 63% 7% 7% 4 1	1800
	11th to 20th	Day	18% 61% 3 9% 8% 2	1902
	21st to end	Day	18% 61% 4 10% 5% 2	1650
March	1st to 10th	Day	29% 29% 5% 16% 14% 7%	4543
	11th to 20th	Day	25% 29% 6% 22% 13% 4	4980
	21st to end	Day	29% 29% 8% 12% 18% 5%	5758
April	1st to 10th	Day	19% 62% 4 7% 6% 2	2400
	11th to 20th	Day	16% 63% 5% 9% 6% 1	2374
	21st to end	Day	16% 63% 4 8% 8% 1	2400
May	1st to 10th	Day	38% 38% 7% 12% 4 1	4162
	11th to 20th	Day	37% 39% 9% 12% 2	4542
	21st to end	Day	20% 40% 9% 18% 12% 1	4907
June	1st to 10th	Day	15% 66% 3 10% 6% 1	2660
	11th to 20th	Day	14% 66% 3 10% 7% 1	2660
	21st to end	Day	12% 64% 5% 10% 8% 1	2660
July	1st to 10th	Day	22% 44% 13% 11% 8% 2	4370
	11th to 20th	Day	26% 39% 12% 15% 7% 1	4365
	21st to end	Day	31% 39% 7% 13% 10% 1	4516
August	1st to 10th	Day	31% 40% 9% 15% 4	4091
	11th to 20th	Day	21% 40% 9% 22% 7%	3777
	21st to end	Day	17% 39% 8% 21% 15% 1	4180
September	1st to 10th	Day	15% 61% 6% 9% 8% 1	2280
	11th to 20th	Day	15% 61% 3 11% 8% 3	2262
	21st to end	Day	23% 60% 3 9% 6%	2071
October	1st to 10th	Day	20% 38% 5% 11% 22% 4	3160
	11th to 20th	Day	11% 38% 8% 14% 22% 7%	3140
	21st to end	Day	9% 36% 8% 20% 22% 4	3350
November	1st to 10th	Day	16% 56% 7% 10% 7% 3	1761
	11th to 20th	Day	14% 57% 3 11% 13% 2	1800
	21st to end	Day	15% 57% 5% 15% 5% 2	1665
December	1st to 10th	Day	26% 36% 11% 16% 8% 3	2600
	11th to 20th	Day	31% 31% 6% 22% 7% 2	2986
	21st to end	Day	30% 28% 10% 16% 14% 2	3960
January	1st to 10th	Night	27% 26% 6% 18% 18% 5%	6349
	11th to 20th	Night	27% 26% 6% 19% 16% 7%	6158
	21st to end	Night	32% 29% 6% 17% 14% 2	6490
February	1st to 10th	Night	17% 62% 6% 10% 3 1	2600
	11th to 20th	Night	18% 60% 2 8% 10% 2	2394
	21st to end	Night	16% 62% 5% 7% 6% 3	1980
March	1st to 10th	Night	31% 29% 6% 18% 7% 9%	5293
	11th to 20th	Night	22% 30% 11% 21% 12% 4	4980
	21st to end	Night	31% 30% 5% 11% 20% 3	5174
April	1st to 10th	Night	19% 62% 2 8% 8% 1	2000
	11th to 20th	Night	16% 63% 5% 8% 7% 1	1975
	21st to end	Night	16% 62% 4 7% 9% 1	2000
May	1st to 10th	Night	36% 39% 7% 12% 4 1	3047
	11th to 20th	Night	38% 39% 10% 10% 3	2576
	21st to end	Night	22% 40% 11% 15% 10% 3	2785
June	1st to 10th	Night	15% 65% 2 9% 9%	1520
	11th to 20th	Night	14% 66% 4 11% 6%	1520
	21st to end	Night	12% 64% 5% 10% 6% 2	1520
July	1st to 10th	Night	25% 40% 15% 11% 8% 2	2480
	11th to 20th	Night	27% 39% 13% 16% 6%	2472
	21st to end	Night	31% 39% 8% 12% 10%	2793
August	1st to 10th	Night	34% 41% 12% 9% 4	2814
	11th to 20th	Night	24% 38% 8% 21% 9%	3084
	21st to end	Night	18% 38% 6% 23% 14% 1	3476
September	1st to 10th	Night	15% 60% 6% 9% 8% 1	1900
	11th to 20th	Night	14% 60% 4 10% 10% 2	1917
	21st to end	Night	21% 60% 3 8% 6%	2104
October	1st to 10th	Night	26% 39% 7% 12% 15% 2	3800
	11th to 20th	Night	10% 37% 6% 18% 24% 4	3779
	21st to end	Night	11% 37% 8% 22% 17% 5%	4196
November	1st to 10th	Night	15% 56% 6% 12% 7% 3	2537
	11th to 20th	Night	16% 56% 3 10% 12% 3	2600
	21st to end	Night	14% 57% 5% 14% 8% 1	2730
December	1st to 10th	Night	23% 36% 8% 21% 9% 2	4580
	11th to 20th	Night	31% 32% 7% 15% 11% 4	5208
	21st to end	Night	34% 28% 8% 13% 15% 1	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	19% 22% 33% 20% 6% 1	200
	11th to 20th	Day	19% 32% 30% 17% 3	240
	21st to end	Day	20% 15% 38% 20% 6% 2	330
February	1st to 10th	Day	21% 21% 33% 19% 6%	300
	11th to 20th	Day	24% 19% 38% 16% 2	441
	21st to end	Day	21% 18% 36% 21% 5%	414
March	1st to 10th	Day	17% 21% 34% 21% 6% 1	500
	11th to 20th	Day	19% 18% 33% 22% 6% 2	500
	21st to end	Day	22% 14% 32% 24% 7% 1	550
April	1st to 10th	Day	22% 16% 35% 19% 7% 1	500
	11th to 20th	Day	18% 15% 31% 23% 11% 2	492
	21st to end	Day	20% 14% 28% 27% 10% 1	500
May	1st to 10th	Day	29% 12% 26% 20% 11% 2	503
	11th to 20th	Day	22% 9% 25% 28% 14% 2	527
	21st to end	Day	17% 6% 20% 25% 22% 9%	549
June	1st to 10th	Day	12% 6% 22% 32% 21% 7%	490
	11th to 20th	Day	14% 4 26% 30% 20% 6%	490
	21st to end	Day	11% 6% 23% 34% 19% 9%	490
July	1st to 10th	Day	12% 3 15% 28% 28% 13%	485
	11th to 20th	Day	7% 1 11% 36% 31% 14%	484
	21st to end	Day	10% 4 17% 33% 28% 9%	532
August	1st to 10th	Day	10% 5% 16% 31% 26% 12%	486
	11th to 20th	Day	12% 3 14% 31% 29% 12%	479
	21st to end	Day	11% 2 19% 29% 24% 15%	539
September	1st to 10th	Day	16% 4 21% 23% 23% 12%	490
	11th to 20th	Day	14% 2 20% 28% 22% 14%	490
	21st to end	Day	26% 5% 21% 27% 15% 6%	490
October	1st to 10th	Day	23% 5% 19% 28% 16% 10%	490
	11th to 20th	Day	13% 11% 27% 25% 20% 4	490
	21st to end	Day	19% 6% 32% 27% 13% 4	516
November	1st to 10th	Day	15% 7% 31% 27% 16% 3	395
	11th to 20th	Day	15% 13% 34% 26% 12%	400
	21st to end	Day	15% 13% 34% 27% 9% 1	265
December	1st to 10th	Day	22% 14% 35% 22% 8% 1	200
	11th to 20th	Day	26% 16% 36% 16% 8%	200
	21st to end	Day	22% 17% 28% 21% 10% 2	220
January	1st to 10th	Night	19% 12% 42% 23% 3	800
	11th to 20th	Night	22% 12% 36% 26% 5%	800
	21st to end	Night	18% 11% 42% 27% 2	880
February	1st to 10th	Night	20% 10% 40% 28% 2	800
	11th to 20th	Night	24% 14% 42% 20% 1	783
	21st to end	Night	20% 13% 42% 22% 3	660
March	1st to 10th	Night	18% 14% 38% 25% 5%	800
	11th to 20th	Night	18% 12% 41% 23% 6% 1	800
	21st to end	Night	19% 8% 41% 26% 5%	880
April	1st to 10th	Night	21% 13% 38% 22% 6%	800
	11th to 20th	Night	16% 10% 33% 34% 6%	791
	21st to end	Night	19% 8% 32% 33% 7%	800
May	1st to 10th	Night	19% 8% 33% 32% 8%	678
	11th to 20th	Night	17% 5% 33% 37% 8%	394
	21st to end	Night	17% 2 20% 39% 21% 2	423
June	1st to 10th	Night	16% 3 22% 41% 18%	380
	11th to 20th	Night	11% 3 23% 46% 17% 1	380
	21st to end	Night	9% 1 20% 51% 18%	380
July	1st to 10th	Night	12% 2 18% 41% 26% 2	370
	11th to 20th	Night	6% 4 12% 48% 29% 2	368
	21st to end	Night	9% 2 15% 45% 26% 2	500
August	1st to 10th	Night	12% 2 11% 46% 27% 3	567
	11th to 20th	Night	14% 3 12% 39% 29% 3	719
	21st to end	Night	14% 3 14% 38% 29% 3	836
September	1st to 10th	Night	15% 3 19% 37% 24% 3	760
	11th to 20th	Night	14% 3 12% 39% 28% 4	760
	21st to end	Night	23% 3 20% 32% 21% 1	760
October	1st to 10th	Night	22% 5% 23% 29% 18% 4	760
	11th to 20th	Night	17% 5% 26% 37% 13% 1	760
	21st to end	Night	19% 6% 31% 32% 12% 1	836
November	1st to 10th	Night	13% 6% 33% 35% 12% 1	780
	11th to 20th	Night	20% 8% 32% 31% 8% 1	800
	21st to end	Night	15% 10% 38% 29% 9%	800
December	1st to 10th	Night	22% 11% 33% 26% 7% 1	800
	11th to 20th	Night	19% 8% 37% 30% 6%	800
	21st to end	Night	20% 9% 36% 32% 3	880

Page generated at 2025-05-23 09:21:03. Version ac07

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